Your space is a bit bigger than mine but I run
http://www.hydrofarm.com/product.php?itemid=8095 this active air 4in 165cfm with carbon filtration. My grow box is in an office/spare bedroom. With my room at ideal temps of 68-74, my grow box keeps around 76-84 with the lights on. When temps in my room got to around 80 degrees earlier in the summer, my box temps soared over 90.
I should have went with the 6in fan and still might reconfigure before next summer.
Edit: If you use the cool tube my understanding is that if you vent the intake from outside the tent you will not ventilate the air inside your tent as the air will be pulled from the outside, through the cool tube and out the vent fan. You could ideally get a large enough cfm fan and use a y-duct and run one for your cool tube and one to vent the tent w/carbon filtration maybe.